expiredNotification is the way to go though.
The message is display in a span with the identifer expiredNotification. So you could check the content (innerHTML) of the span and if that's set to "Your password is successfully updated." then you replace the content with what you wish. Note that would work only for the language you are comparing it to.
Something along these lines:
var expiredNotification = document.getElementById('expiredNotification');
if (expiredNotification)
{
if (expiredNotification.innerHTML == 'Your password is successfully updated.' )
{
expiredNotification.innerHTML = "Your new password is preeettttty cool!" ;
}
}